1. Field of the Invention
This invention involves a system and a method for imaging a portion of a human body using ultrasound.
2. Description of the Related Art
Viewed most generally, a diagnostic ultrasound imaging system works by transmitting a set of ultrasonic signals into a portion of a patient's body, sensing the resulting echo signals, and then interpreting the echo signals to form an image. A goal of all diagnostic ultrasonic imaging systems is to increase the clarity and resolution of the image so that more diagnostic information will be available to the physician. To this end, many different methods have been proposed for generating the transmission signal so that is has certain advantageous properties, for filtering the echo signals to eliminate certain unwanted components, or both.
One method that has recently shown great promise involves transmitting a pair of complex, amplitude-modulated ultrasonic pulses into the body in such a way that the second pulse is the time-domain "inverse" of the first. The second pulse, which is applied to the body as soon as possible after receiving the echo from the first transmitted pulse, can therefore be viewed, in one common representation of the two signals, as lying 180 degrees out of phase with the first pulse. The system then sums the time-domain return signals from each of the two transmitted pulses.
This technique is known variously as "pulse inversion," which accurately describes the relationship between the input pulses, "phase inversion," or even "harmonic imaging," since certain harmonics (usually the second) of the input signals are isolated. The advantage of this arrangement is that it isolates echo signals from body structures whose response is non-linear: Echo signals from body structures with a linear response will, by definition, in theory "cancel" each other upon summation, whereas echo signals from non-linear structures in general will not. U.S. Pat. No. 5,632,277 (Chapman, et al., issued May 27, 1997) for "Ultrasound Imaging System Employing Phase Inversion Subtraction to Enhance the Image;" and U.S. Pat. No. 5,706,819 (Hwang, et al., issued Jan. 13, 1998) for "Ultrasonic Diagnostic Imaging with Harmonic Contrast Agents" both describe such systems, although Hwang, et al., fail to realize that the pulse-inversion technique may be used to image tissue as well as micro-bubble contrast agents.
As always, what is needed is a system that discriminates between and images body structures even more clearly than what is now possible. This invention provides such a system.